The present invention relates to a high frequency power generator particularly for use in high frequency surgery.
1. Field of the Invention
High frequency power at high output is needed in various industrial, medical or scientific applications. As examples of industrial applications, drying appliances operating by heating a material through dielectric losses in a high frequency field, or smelting apparatus in which the heating of a material to be molten is effected by means of eddy currents in a high frequency field may be mentioned. In scientific work, high frequency power is needed for the excitation of a plasma, for example in spectral analysis or nuclear fusion reactors. In medicine, high frequency power with high output is needed in high frequency surgery, for example.
In many applications the load on a high frequency power generator is not constant, but often subject to very large and sudden changes. For example, in high frequency surgery the impedance of a surgical probe connected to a high frequency power generator can vary between about 50 ohms and 4 kiloohms within milliseconds. Even the reactive component of a load impedance may vary rapidly within wide ranges. Thus, very fast changes of the reactive component of a load impedance may occur when a surgical probe is rapidly removed from the tissue being cut.
It is common to most applications of high frequency power generators that the effect to be caused by means of the high frequency power does not depend upon strict maintenance of the frequency value. In high frequency surgery, for example, frequencies between 300 kHz and 2 MHz are commonly used. In this, it is not of substantial consequences when, for example, a deviation from the frequency range set in any particular case amounts to about 10% in either direction. In high frequency smelting furnaces the frequencies used may even be between 200 kHz and 27 MHz.
At the same time, the applications just described require generators to have a high efficiency. For example, ventilators for cooling a high frequency surgical instrument are not desired in an operating theatre, because the flow of cool air could whirl up bacteria. With an available high frequency output of 400 W, a high frequency power generator must have an efficiency of at least 90 percent, for example, in order that the instrument will not overheat.
To attain these required high efficiencies, it is nowadays the practice for semiconductor amplifier components that operate in a switching mode to be used in the power generators. At present most high frequency surgical instruments operate on this principle.
Because of the switching operation, harmonics are generated in the frequency spectrum of the output current, this having various disadvantages. In high frequency surgery the harmonics of much higher frequency than the actual operating frequency may cause high capacitive current leaks which present a risk of the patient becoming burnt. Moreover, the unavoidable scattering components of the circuit produce strong overshoot pulses which endanger the operational safety of the amplifier components by causing overvoltages. Furthermore, unlimited efficiencies cannot be attained with switching amplifiers because of the switching delay of the amplifier components.
2. Description of Related Art
From the document DE-A-38 05 921, for example, it is known to employ power amplifiers having transistors connected as a quasi-complementary stage and functioning in switching operation. Connected in series to the output of this power amplifier is a series resonant circuit which conducts the output current to a grounded parallel resonant circuit connected in parallel. The series resonant circuit connected in series and the parallel resonant circuit connected in parallel are each tuned to the mean operating frequency.
The series resonant circuit makes it possible for the output current of the power amplifier to be almost completely sinusoidal.
With the parallel resonant circuit, the load impedance at the operating frequency is brought to a real value, in order substantially to avoid phase shifts between current and voltages as well as reactive currents.
From the description of the high frequency power generator in DE-A-38 05 921 it is also known that the drive voltage of the power amplifier is supplied by an auxiliary oscillator at the instant of stimulation of oscillation, and that following the stimulation of oscillation the drive of the power amplifier is changed over in such a manner that the drive voltage of the power amplifier is then generated through a current regeneration or feedback when the magnitude of the load impedance connected to the high frequency power generator falls below a characteristic preset value, and through a voltage regeneration or feedback when the opposite is the case. Furthermore, according to DE-A-38 05 921 an electronic switch is provided for enabling a change-over between the two possibilities of feedback even during operation, when with fluctuating load impedances the preset value is exceeded or fallen short of.
However, the high frequency power generator described in DE-A-38 05 921 has the disadvantage of being equipped with a relatively large number of components, and also of the stability of the circuit not being assured under capacitive loads.
In high frequency surgery it is possible to achieve coagulation with good reproducibility by applying a constant output voltage. It is therefore of advantage for the high frequency generator to produce an output voltage that is independent of the load as far as possible. Usually the output voltage of the high frequency generator is controlled through the d.c. supply voltage of the generator. In order for this to function well, the high frequency power generator should have a transformation ratio of the output voltage to the d.c. supply voltage that is constant and, as far as possible, independent of the load conditions.
With the arrangement shown in DE-A-38 05 921, conditions may arise under capacitive load which causes the output filter to be so out of tune that a stable current feedback condition can occur at several frequencies. However, only one frequency has the desired property of the transformation ratio remaining constant. At the other frequencies a high overvoltage may occur, which may have detrimental effects such as the patient becoming imparted with undesirably heavy necroses on the edges of a cut.